1 B.Rarely to be seen 強調的是壹種“目的”概念，在這裏與句子的邏輯不符合，所以是錯的。
2 C.Rarely seen是正確的答案，做句子主語 the white mountain goat 前置定語，具體解釋就是： The white mountain goat, WHICH IS RARELY SEEN, is an extremely sure-footed - - - .
3 如果是Rare to be seen的話，也是對的，這樣就等於：WHICH IS RARE TO BE SEEN.